(1) Field of the Invention
The present invention relates to a control valve for a variable displacement compressor, and more particularly to a control valve for a variable displacement compressor which is suitable for controlling discharging capacity of refrigerant of a variable displacement compressor for an automotive air conditioner.
(2) Description of the Related Art
A compressor used in a refrigeration cycle of an automotive air conditioner is driven by an engine whose rotational speed is varied depending on a traveling condition of the vehicle, and hence is incapable of performing rotational speed control. For this reason, in general, a variable displacement compressor capable of changing discharging capacity of refrigerant is employed so as to obtain an adequate refrigerating capacity without being constrained by the rotational speed of the engine.
In the variable displacement compressor, in general, a wobble plate disposed within a crankcase formed gastight, such that the inclination angle thereof can be changed, is driven by the rotational motion of a rotational shaft, for performing wobbling motion, and pistons caused to perform reciprocating motion in a direction parallel to the rotational shaft by the wobbling motion of the wobble plate draw refrigerant from a suction chamber into associated cylinders, compress the refrigerant, and then discharge the same into a discharge chamber. In doing this, the inclination angle of the wobble plate can be varied by changing the pressure in the crankcase, whereby the stroke of the pistons is changed for changing the discharge amount of the refrigerant. The control valve for a variable displacement compressor provides control to change the pressure in the crankcase.
In general, such a control valve for a variable displacement compressor, which variably controls the discharge capacity of the compressor, operates to introduce part of refrigerant discharged from the discharge chamber and having discharge pressure Pd, into the crankcase formed gastight, such that pressure Pc in the crankcase is controlled through control of the amount of refrigerant thus introduced, which control is carried out according to suction pressure Ps in the suction chamber. That is, the control valve for a variable displacement compressor senses the suction pressure Ps, and controls the flow rate of refrigerant introduced from the discharge chamber into the crankcase at the discharge pressure Pd, so as to hold the suction pressure Ps at a constant level.
To this end, the control valve for a variable displacement compressor is equipped with a pressure-sensing section for sensing the suction pressure Ps, and a valve section for causing a passage leading from the suction chamber to the crankcase to open and close according to the suction pressure Ps sensed by the pressure-sensing section. Further, a type of the control valve for a variable displacement compressor which is capable of freely externally setting a value of suction pressure Ps to be assumed, at the start of the variable displacement operation, is equipped with a solenoid that enables configuration of settings of the pressure-sensing section by external electric current.
By the way, conventional control valves for a variable displacement compressor which can be externally controlled include a type for control of a so-called clutchless variable displacement compressor configured such that an engine is directly connected to a rotational shaft without providing a solenoid clutch between the engine and the rotational shaft on which a wobble plate is fitted, for execution and inhibition of transmission of a driving force to the engine (see e.g. Japanese Unexamined Patent Publication (Kokai) No. 2000-110731 (Paragraph numbers [0010], [0044], and FIG. 1)).
This control valve comprises a valve section causing a passage communicating between a discharge chamber and a crankcase to be opened and closed, a solenoid for generating an electromagnetic force causing the valve section to operate in the closing direction, and a pressure-sensing section for causing the valve section to operate in the opening direction as suction pressure Ps becomes lower compared with the atmospheric pressure, which are arranged in this order. Therefore, when the solenoid is not energized, the valve section is in a fully open state, whereby pressure Pc in a crankcase can be held at a pressure close to discharge pressure Pd. This causes the wobble plate to become substantially at right angles to the rotational shaft, enabling the variable displacement compressor to operate with minimum capacity. Thus, the discharging capacity of refrigerant can be substantially reduced to approximately zero even when the engine is directly connected to the rotational shaft, which makes it possible to eliminate the solenoid clutch.
However, the conventional control valve for controlling a variable displacement compressor having no use for the solenoid clutch is configured such that the pressure-sensing section and the valve section are arranged with the solenoid interposed therebetween, and the suction pressure Ps is introduced to the pressure-sensing section which compares the suction pressure Ps and the atmospheric pressure, via the solenoid. This necessitates the solenoid in its entirety to be accommodated within a pressure chamber, and hence components of the solenoid need to be designed with considerations given to resistance to pressure.
To eliminate this inconvenience, the present applicant has proposed a control valve for a variable displacement compressor configured such that the plunger of a solenoid is divided into a first plunger and a second plunger, and a pressure-sensing member, such as a diaphragm or a bellows, is interposed therebetween for sensing suction pressure, whereby the valve lift of a valve section for controlling pressure in a crankcase is controlled by the second divisional plunger (Japanese Unexamined Patent Publication (Kokai) No. 2003-289581).
More specifically, for example, as shown in FIG. 8, the control valve 101 for a variable displacement compressor includes a body 102 that accommodates a valve section 110 and a solenoid 120, and a core 121, a first plunger 122, and a second plunger 123, which form the solenoid 120, are arranged in series within the body 102. Between the valve section 110 and the solenoid 120 within the body 102, there is disposed a holder 131 formed of a magnetic member in which the second plunger 123 is axially movably disposed.
The second plunger 123 has a non-magnetic guide 133, which is formed e.g. of polytetrafluoroethylene and has low sliding resistance, provided on the periphery thereof. The outer peripheral surface of the guide 133 is in sliding contact with the inner wall of the holder 131, whereby when the second plunger 123 is axially moved forward and backward, the guide 133 serves to guide the second plunger 123, while maintaining the same at a predetermined distance from the inner wall of the holder 131. The guide 133 has a circumferential part thereof cut open, thereby allowing suction pressure Ps to be introduced into a space formed on a lower end face of the second plunger 123.
Further, the second plunger 123 has an annular flange portion 124 assembled therewith such that it is fixed at an upper end location thereof, and a spring 161 is interposed between the flange portion 124 and an upper end face of the holder 131. A shaft 113 of the valve section 110, which is axially movably disposed within the body 102, has a lower end thereof in abutment with the second plunger 123 at an upper axial location of the second plunger 123.
The spring 161 urging the second plunger 123 upward is configured to have a larger spring force than that of the spring 162 urging the valve element 111 of the valve section 110 in the valve-closing direction. Therefore, when the solenoid 120 is not energized, the valve element 111 at the end of the shaft 113 is moved away from a valve seat 115 formed inside the body 102, and the valve section 110 is in its fully open state.
Below the second plunger 123, a pressure-sensing member 151 (diaphragm in the figure) constituting a pressure-sensing section is disposed. The pressure-sensing member 151 has its outer peripheral edge sandwiched by the holder 131 and a casing 141 of the solenoid 120 forming part of the body 102. Thus, part forming a pressure chamber of the control valve 101 for a variable displacement compressor extends up to a portion partitioned by the pressure-sensing member 151, and part lower than this portion receives the atmospheric pressure.
Within the casing 141, a solenoid coil 142 is disposed, and inside the solenoid coil 142 is disposed a sleeve 143. The sleeve 143 has a core 121 inserted into a lower portion thereof and fixed thereto. Between the core 121 and the pressure-sensing member 151 is disposed a first plunger 122 such that the first plunger 122 is axially movable within the sleeve 143. The shaft 126 disposed along the axis of the first plunger 122 has an upper end thereof inserted into the first plunger 122 for connection between the shaft 126 and the first plunger 122, with the lower end of the shaft 126 being supported by a bearing member 135 disposed within the body 102. Disposed between the bearing member 135 and a flange portion 127 fitted on the periphery of the shaft 126 is a spring 163 which urges the first plunger 122 toward the pressure-sensing member 151.
Due to the arrangement described above, the pressure-sensing member 151 fluidically separates a space having the first plunger 122 disposed therein and a space having the second plunger 123 disposed therein from each other. In other words, a section extending from the valve section 110 to a portion where the pressure-sensing member 151 is disposed, including the second plunger 123 which controls the valve lift of the valve section 110, is formed as a block to which pressure is applied, and the solenoid 120 exclusive of the second plunger 123 is not accommodated in the pressure chamber, allowing the same to be configured to be open to the atmosphere. Moreover, the second plunger 123 which controls the valve lift of the valve section 110 is urged in a direction away from the pressure-sensing member 151, so that when the solenoid is not energized, displacement of the pressure-sensing member 151 is not transmitted to the valve section 110, and at the same time the valve section 110 is held in its fully-open state, thereby enabling the variable displacement compressor to be controlled to the minimum displacement.
The first plunger 122 and the second plunger 123 formed by dividing the solenoid 120 are separated from each other when the solenoid is not energized, whereas when the solenoid is energized, they are attracted to each other to behave as one plunger. Therefore, when the solenoid 120 is energized, control is performed by the one plunger which is formed by the first plunger 122 and the second plunger 123 integrally attached to each other. Since the pressure-sensing member 151 is disposed between the first plunger 122 and the second plunger 123, opposed surfaces of the first plunger 122 and the second plunger 123 are formed to have a planar shape. As a result, when the solenoid is energized, a magnetic circuit is formed between the flat opposed surfaces whereby the first plunger 122 and the second plunger 123 are attracted to each other with the pressure-sensing member 151 being interposed therebetween.
In the arrangement described above, the magnetic circuit of the solenoid 120 surrounding the solenoid coil 142 is formed by the core 121, the first plunger 122, the second plunger 123, the holder 131, the casing 141, and the like. In this case, the magnetic circuit is formed in a state where the second plunger 123 is inserted into the holder 131, so that the attractive force of the solenoid 120 acts on the second plunger 123 in the radial direction thereof. Therefore, when the attractive force is large, there is a fear that the axial motion of the second plunger 123 is obstructed, or undesired vibrations of the second plunger 123 are caused by the attractive force. To overcome the inconveniences, conventionally, as shown in FIG. 8, the second plunger 123 is configured to have the non-magnetic guide 133 provided on the periphery thereof such that the guide 133 is in sliding contact with the inner wall of the holder 131, or a non-magnetic sleeve (not shown) is interposed between the second plunger 123 and the holder 131. However, the provision of such a non-magnetic member raises the problem that the number of component parts is increased to increase manufacturing costs.
Further, since the radial attractive force is applied to the second plunger 123 from the holder 131, an attractive force in the axial direction, which is the operating direction of the second plunger, is weakened, and especially when the first plunger 122 and the core 121 are away from each other, it is difficult to obtain sufficient attractive force characteristic.
Furthermore, in the arrangement in which the second plunger 123 slides on the holder 131 as described above, the problem of hysteresis becomes serious in which in spite of the control position being set to the same position, the valve element 111 is controlled to positions, different between when the valve element 111 is opened and when the valve element 111 is closed, e.g. due to the influence of a frictional force caused by the sliding motion.